Automatic volume control circuit



Oct. 26, 1937. G. L. BEERS AUTOMATIC VOLUME CONTROL CIRCUIT AlllA VV" lNvE NToR" 5.00m H m MODA m). m.

G. L.. BEERs 2,096,874

AUTOMATIC VOLUME CONTROL CIRCUIT Filed March 16, 1954 2 Sheets-Sheet 2 oct. 26, 1937.,

Il :Il I

INVENTOR GEORGE L. BEERS BY j MvL/L/ ATTORNEY Patented Oct. 26, 1937 UNITED STATES AUTOMATIC VQLUME CONTROL CIRCUIT George L. Beers, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 16,

22 Claims.

My present invention relates'to gain control circuits for receivers, and particularly to radio receivers of the automatic volume control type utilizing background noise suppressor arrangements.

One of the main objects of this invention may be said to reside in a method for improving the selectivity of the background noise suppressor arrangement of a radio receiver, and to additionally provide the suppressor with an on or o characteristic with conventional tubes.

Another important object of the invention is to provide in a receiver equipped with an automatic volume control network andan inter-station noise suppressor tube, a plurality of tuned circuits and a double diode which are used to control the suppressor tube, and the difference' urne control network, the characteristic being obtained through a feed back arrangement which utilizes an auxiliary tube so yconnected that as soon as a little signal potential is appliedV to theV detector the tube increases the negative bias on the noise suppressor tube and permits more signal to get through.

Another object is to provide an automatic oontrol system for a superheterodyne receiver which performs the functions of automatic volume control, inter-station noise suppression and regulation of the speed of eicctiveness of the noise suppressor, the latter function being accomplished by the second detector tube, or by an auxiliary tube independent of the second detector.

Still other objects of th-e invention are to improve generally the eiiiciency of automatic volume control systems for radio receivers, and particularly to provide an improved volume controlv system which is not only reliable in operation, but is readily manufactured and assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims. The invention itself, both as to its organization and method of operation, will best be understood by reference to the following description taken in connection with the drawings, in which I have indicated 55 diagrammatically several circuit organizations a radio receiver equipped with an automatic vol-V 1934, Serial No. 715,870

(CL Z50- 20) carried into effect..

receiving system of the superheterodyne type,

the system including a quiet Yautomatic volume control arrangement, The latter type of receiver instrumentality is used because one of the diiiiculties encountered in the operation oi a radio L receiver provided with a conventional automatic volume control system is the high level of noise reproduced when tuning between stations. is due to the factrthat an effective automatic volume control tends to keep the signal voltage at the second detector Vconstant irrespective of the received signal strength.

This

This characteristic of an automatic volumeV control also gives the effect that when tuning in a strong signal from a local transmitter the Vreceiver appears to lack selectivity. lIhe result of ceiver inoperative until a signal of a desired Y strength has been tuned in jhave been incorporated in a number of modern broadcast re-` ceivers. In some of these additional selectivity has been used between the automatic'volume control and noise suppression systems. Through the use of this latter arrangementy the selectivity of the noise suppression system can be made such that no sound will be -heard from the receiver` until it is within one or two kilocycles off resonance with the desired signal. 'I'he chief disadvantage of such a system is that the additional selective circuit is independent of thercircuits which feed the signal to the second detector,

and, if the alignment of this circuit with the other circuits of the receiver is destroyed, it then becomes impossibleto tune the receiver properly. It is the ultimate goal of the arrangements shown in the present disclosure to improve the selectivity characteristic of a noise suppression system, and, in addition, to provide a method of giving such a system an on and olf characteristic while using conventional tubes. Considering, then, the circuit arrangement in Fig. l it Will be observed that the receiver comprises a signal collector, such as a grounded -antenna circuit, which is followed by a tunable radio frequency amplifier 2 of conventional design, a first detector ,3 of any Well known construction, and an intermediateV frequency amplifier network 4, 4. j

The network 3 is fed with locally produced oscillations from the local oscillator L. O. The

oscillator and first detector may be coupled in' any desired /manner well known in the art. Of course, any other type of frequency changer may be used. For example, the local oscillator and first detector may be embodied ina single tube.

The tuning devices of the amplifier' 2, local oscillator and frequency changer 3 are uni-controlled, and the dotted lines' are to be understood as designating the uni-control tuning means for the tuning condensers of the signal and local oscillation circuits. The transformer M has its primary and secondary windings tuned to the operating intermediate frequency. The tuned secondary circuit 5 is connected to the input electrodes of the amplifier 4. The coupling transformer M connects the tuned output circuit of tube 4 to the input electrodes of intermediate frequency amplifier The input circuit of tube 4' is coupled, through a signal path including condenser 6, to the input circuit of the volume control amplifier D. The latter functions to amplify the intermediate frequency energy fed from the input circuit of the intermediate frequency amplifier 4 to the automatic volume control and suppressor circuits. condenser E serves to supply bias voltage to the signal input grid of tube D.

The anode of tube 4 is connected to the tuned primary circuit of the coupling transformer M1. The tunedA secondary circuit 8 is connected between the input electrodes of the second detector ytube 9, the tuned windings of the coupling M1 being, of course, resonant to the operating intermediate frequency. The anode potential for tube 9 is derived from the voltage supply network of the receiver, the audio potential component of the demodulated signal current developed across load resistor I I being transmitted .through coupling condenser I0 to one or more stages of audio amplification, a loud speaker (not shown) following the nal audio amplifier stage. The voltage supply network includes the two bleeder resistors P-P' connected in parallel.

The letters G and G designate the ground points on the bleeder resistors, and one junction of the resistors is connected to the positive side of the supply line, while the other junctionris connected to the negative side. The two resistors in actual practice may be one.

The automatic volume control network is of a type disclosed and claimed by W. L. Carlson, et al., in application Serial No. 648,422, filed December 22, 1932. Such a network comprises a control tube A of the type, also known as a duplex diode triode. The latter comprises within a common tube envelope atriode section and The resistor 6 in series vir/ithV a pair of diode sections, all sections having a common cathode and the diode anodes being outside the electron stream to the grid and plate of the trode section. Such a tube is described and claimed by T. M. Shrader in application Serial No. 622,140, filed July 12, 1932, Patent No. 2,058,834 of October 27, 1936. The diode anodes of tube A are tied together, the anodesA being connected through lead I3 and condenser I3 to the anode side of the tuned output circuit I4 of The plate of tubefA is connected by lead 50 to a positive potential point on the resistor P', the athode lead returning to the negative side of the bleeder through ground. A resistor R1 is inserted in the cathode lead olf tube A to maintain the cathode at a predetermined positive potential abo-ve ground when signals below a desired intensity level are received. Between the diode anodes of tube A and the ground Vside of its cathode are connected three resistor sections Rz, R3, R4. in series. The grid of the amplier 2 is connected to the junction of resistors R2 and R3 through lead 5I, while the signal grids of the tub-es 3 and 4 are connected tothe junction of resistors Rs and R4. These connections are the automatic volume control connections to the radio amplier first detector and intermedi-ate frequency amplifier 4, and the symbol AVC is used to designate the connections.

Current will flow through the series resistive path R2-R3-R4-R1 only when the diode anodes of tube A are positive with respect to the cathode. As long as signals below a predetermined intensity level are impressed upon the second detector 9, the grid of tube A is at a minimum negative potential, and the space current flow through resistor R1 is a maximum. In this condition two things happen. First, the cathode of tube A is positive with respect to the diode anodes thereby preventing flow of current through the diode circuit. This results in a delayed automatic volume control action since theY controlled tubes are at maximum sensitivity in this condition. Secondly, and by virtue of the direct current connection I5 between the cathodes of amplifier 4 and second detector 9 and the cathode side of resistor R1, the intermediate frequency amplifier and second detector are biased substantially to cut-off to prevent the transmission of undesired background noise impulses to the audio amplifier network. If the noise suppressor connection (designated ANS) Vwere not used, the noises would be greatly amplified due to the controlled ampliers operating at maximum sensitivity in the no-signal condition.

The control circuit for the grid of tube A includes a tube B, and the latter includes a cathode and a pair of diode anodes I', 2'. The anode I' is connected to the cathode through a path which includes in series the tuned circuit 20 and the resistor R7 havinga value of about 1 megohm, a radio frequency by-pass condenser 2l, of about 0.001 mfd., being connected between the negative s ide of resistor R1 and ground. The anode 2 is connected to the cathode of tube B through a path which includes in series the tuned circuit 22, the resistor R5 and resistor Re, the latter two resistors having values of 200,000 ohms and 250,- 000 ohms respectively. The junction of resistors R5 and Re is connected to the anode of tube C whose function will shortly be described, a radio frequency by-pass condenser 23 of 0.001 mfd. being connected to ground from the said junction.

nected'to the anode side of resistor R7.

arrangement shown in Fig. 1 the difference inthe s Va functionrof the strength of `the signalf If ,theN

The tuned circuits I 4, 22'andf2ll are coupled in cascade, and are all tuned to the operating intermediate frequency. The grid of tube A is conn In-the selectivity preceding two parts of the'noise s up-y pression system.l determines the frequency band over which the device will o-perate. is accomplished by causing the part ofthe system which has the least selectivity to oppose the-part which has Vthe greatest selectivity. Tube B is a double diode which controls the grid bias of tube A. Diode I is preceded by the greatest selectivity, and when a signal is applied causes the bias of tube A to become more negative.

Diode 2 is preceded by less selectivity than diode l,rand a signal impressed on this diode causes the grid bias of tube A to become vmore positive. If the Vselectivity and gain preceding each of the diodes was the same and the same resistance was used in each circuit, the Vgrid bias of tube A would not change irrespective of the signal strength. However, the selectivity ahead of diode I is greater than that ahead of diode 2 and the amount of change in bias of tube A provided by each diode can be controlled through the use of the two resistors R5 and Rt included in the circuit with diode 2'. The frequency band over which the grid bias of tube A is caused to become negative is determined by the dilference in the selectivity of the circuits ahead of the two diodes, and the relative signal voltage applied to each diode of tube B.

The operation of the control system up to the present point will now be described. When the receiver is detuned from the desired signal the vbias on the grid of tube A is zero, and the intermediate frequency amplier tube ll and second detector 9 are biased so as to be inoperativeby the voltage drop across the resistor R1 in the cathode circuit of tube A. As the receiver is tuned towards resonance with the desired signal a comparatively large signal voltage is applied to diode 2 while the signal voltage on diode I is still negligible.

The signal app-lied to diode 2 causes the bias of tube A to become positive. As the receiver is tuned nearer to resonance with the signal, the

, signal applied to diode 2 reaches its maximum value, and the signal applied to diode l becomes of such a value that the negative bias derived from this diode offsets some of the positive bias derived from diode 2.

When the receiver is tuned to Within two or l' three kilocycles off resonance the bias derived from diode I becomes greater than that from diode 2' so that the efectivebias on tube A becomes negative reducing its plate current to a low value, and causing the intermediate frequency amplier 4 and the second detector 9 to operate in a normal manner. The practical effect of this system is to cause the frequency band width, over which sound will be heard as the receiver is tuned through a desired station, to be which sound can be heard as the receiver is tuned through resonance with a desired signal is ThisV result f signal strength is 1,000 times the normal input C of thereceiver the frequency bandwidth over which the noise suppressor will operate willbe the frequency band width of the receiver selectivity characteristic at 1,000 times normal Vinput which for someY receivers is approximately 25jkilocycles.' For a signal strength of twice normal input the frequency band width will be approxi-v mately 6 kilocycles.

, The relative value of resistors Rs and R5 de-K,

termine themagntude of the positive bias ptotential-applied to the, grid of tube A.V This pto-l tential must be so adjusted that when the receiver is tuned to resonance, the negative bias potential derived from diode l is greater than the 8 of the second detector 9 through a lead 30" and condenser 30. The grid and cathode of tube C are maintained at diiferent potentials by connecting the cathode lead of tube C through lead 53 to a point on resistor P', the grid of the tube being connected through resistor R10 to a point on bleeder'P' which is negative with respect to the cathode point. The plate of tube C is' connected through resistor R11, having a value o f 500,000 ohms, to the cathode side of resistor R1.

The function of the* tube C is to impart anv on or off characteristicto the noise suppression ar-l rangement without the use of tubes of special construction. It will be noted kthat the 500,000 ohm resistor R11 in the plate circuit of tube C is also included in the grid circuit of tube A. An Yincrease in the current flowing through this Vresistor causes an increase in the negative grid bias of tube A. The grid of tube C is connected to the grid of the second detector through a cou# pling condenser. Therefore, the signals which are applied to the grid of the second detector are likewise applied to the grid of tube C. Tube C is biased so that its plate current when no signal is applied is negligible.

As a signal is tuned in the grid bias `of tube vA derived from diodes lV andV 2 finally becomes 'slightly negative. This reduces the plate current flowing through theV resistor R1 in the cathode circuit of tube A, and thus causes a corresponding reduction in the negative bias applied to the intermediate frequency amplifier tube, which feeds signalsto the second detector, as well as a reduction in the bias applied to the second detector. 'I'his allows a signal of small Vamplitude to be applied to the second detector 9 andY tube C. The signal applied to tube C causes an increase in its plate current which in turn causes anincrease in the .negative bias of tube A, thus permitting a signal of greater amplitude to reach tube C.Y

In other words, a feed back system is emA ployed so that as soon as the slightest signal is permitted to reach the second detector'the bias of the intermediate frequency amplifier tube 4" feeding the second detector returns to its normal value. With a system of this type having an on or oif characteristic, the bias of the second 'dee tector can be controlled simultaneously with the intermediate frequency amplier land. thereby provide a greater degree of control. Considerable distortion would result at certain signal levels if the systems of the past were used to control the bias of the seconddetector.

In cases where it is desired to combine the function of demodulation With that of tube C the modified circuit shown in Fig. 2Vmay be employed. Only those portions of the circuits associated with tubes 9 and C which have been modified areV shown in the interests of simplicity of description. The tube C may be employed to demodulate the intermediate frequencysignaland impart the on or off characteristic to the noise suppressor. The tube C' is a diode whose anode is connected to its cathode through tuned circuit 8 arranged in series with load resistor 4|, the latter being shunted by a radio frequency by-pass condenser 42. Y

The anode side, or negative terminal, of-resister 4| is connected by lead 40 to the junction of resistors R5 and Re. The audio coupling condenser I0 is connected to the negative side of resistor 4|, the resistor R1 in the cathode circuit of tube A being connected at its ungrounded end to the cathode side of resistory 4|.

'I'he operation of this modification will be clear from the aforegoing description of the arrangement in Fig. 1, as well as the following explanation. As soon as signals are impressed on tuned circuit 8 the current flowing through the circuit of diode C' develops a voltage across resistor 4|.

'The direct current potential component is used to negatively bias the grid of tube A as described in Fig. 1. This results in a decreased voltage drop across resistor R1, and a corresponding reduction of the bias on tube 4. In this way still more signals are permitted to be impressed on the second detector diode. As signals decrease in intensity the reverse action takes place.

In the aforegoing description it was pointed out that noise'suppressor systemsof the type which required the use of additional selective circuits are subject to the limitation that if the selective circuit for any reason becomes detuned, the noise suppressor will then cause the user of the receiver to tune it to one side of the desired carrier wave. The arrangement shown in Fig, 1 does not avoid this difficulty, since tuned selective circuits which are not in the signal channel are used to apply the signal to the noise suppressor. In Fig. 8 there is shown a modified form of the invention, wherein the selective circuits which control the operation of the noise suppressor are the same circuits Which are used to supply the signal to the second detector 60. This arrangement, therefore, avoids the aforementioned limitation.

In the circuit arrangement shown in Fig. 3 the element of the receiver system which is controlled is the audio amplifier tube 6|. The radio frequency feed back system shown in Fig. 1 is not utilized in this embodiment of the invention. The intermediate frequency amplifier 62 has its tuned inputcircuit coupled to the first detector of the system through the tuned coupling means 63, and the tuned coupling transformer 64 couples the second detector to the plate circuit of the amplifier 62. The second detector is shown as of the diode type, the audio component of the demodulated signal energy being impressed upon the grid of audio'tube 6I through an audio coupling condenser 65.

The anode side of load resistor 66 isconnected to the coupling condenser 65, and the same side of resistor 66 is connected to the junction of resistors 61 and 68 through a resistor 69. 'I'he duplex diode triode tube A has its grid connected to the plate of diode tube B' through the resistors 61', 61 and 68, the cathode of the diode tube being connected to the grid side of resistor 68. The function of resistor 61 (which has a value of 2 megohms, is to limit the grid current which tube A will take when its bias becomesfpositive. The diode plates of tube A are strapped together, and connected, through condenser 10, to the high potential side of the plate circuit of tube 62; while condenser 1| connects the plate of diode B to the same point. Y

The cathode of tube A is connected to ground through the resistor 12, and the cathode side of the resistor is connected by a lead 13 to the cathode side of resistor 66, and by a lead 14 to the cathode of audio amplifier 6|. The grid of tube 6| is connected to ground through resistor 15. The automatic volume control circuit is provided by connecting the strapped diode anodes of tube A to the grounded side of resistor 12 through the three resistor sections 16, 11 and 18 arranged in series. The connections from these resistor sections to the controlled stages are the same as described in connection with Fig. l. The plate circuit of audio tube '6| is connected to the audio output system, while the bleeder resistor P1 provides the necessary potentials for the electrodesV of the various tubes.V

The operation of the modification shown in Fig. 3 Will now be explained. As a signal is tuned in the signal potential applied to diode B is greater than that applied to the second detector, due to the difference in the selectivity of the circuits preceding these two diodes. The bias on the control grid of tube A then becomes slightly positive. As the receiver is tuned more nearly into resonance with the desired signal, the signal applied to the second detector increases to the point where the negative bias obtained from the drop across ,resistor 66 equals the positive bias derived from resistor 68. When tuned to within two or three kilocycles off resonance the drop across resistor 66 exceeds that across resistor 68, and the control grid of tube A becomes negative with respect to its cathode thereby reducing the plate current flowing through resistor 12. When this current is reduced to a reasonably low value the bias on tube 6| returns to the normal value and the complete receiver then functions in the usual manner. tion as the Ydiode 2 in Fig. 1.

The effect of the Vselectivity characteristics of the tuned circuits of transformer 64 may be best understood by reference to Fig. 4 Vwhich shows ie relative voltage obtained across the primary and secondary of this transformer as the frequency is varied. As is typical of coupled tuned circuit transformers, there'is a pronounced dip in the voltage developed across the primary at the center of the frequency band passed by the transformer. The voltage developed across the secondary is a maximum at this point. This characteristic is made use of in the differential noise suppressor arrangement, since the voltage which tends to give the noise suppressor tube a positive bias diminishes at the point of resonance, whereas the voltage which biases the control grid in a negative direction is a maximum Vat this point. This same characteristic may be utilized in the arrangement shown in Fig. Y1.

While I have'indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular The diode B performs the same func- ,potentials in response to an incoming carrier wave, and means for differentially utilizing said potentials to render ineffective the action of the gain-minimizing means.

2. The invention set forth in claim 1, characV terized in that the uni-directional potential deriving means is constituted by a plurality of devices, and further characterized in that means are provided whereby the selectivity ahead of one of said devices is more than the selectivity ahead of the other of said devices, whereby the said potentials appear sequentially as the tuning of the receiver approachesV resonance with the carrier wave of a desired station. A Y

3. The invention set forth in-claim1', characterized in that means vare provided whereby one of said potentials tends yto reinforce the action of the gain-minimizing means and wherebyl the other of said potentials tends to nullify the action of the Ygain-minimizing means, and further characterized in that selective means are provided whereby the potentials appear in the order given as the tuning'of the receiver approaches resonance with the, carrier wave of a'desired station.

4. The invention set forth in claim 1, additionf ally characterized in that energy feed-back means are provided for regeneratively increasing the differential between the said uni-directional potentials whereby there is a sharp increase Ain the rate at which the minimizing means is lrendered ineffective as a signal is tuned in.

5. In a radio receiver, a signal receiving and amplifying channel havingfa predetermined degree of selectivity, means for rendering inoperative an element in said channel during the Yabsence of an incoming SignaL'means for utilizing opposing the action of the rst named means.

6. In combination, means for deriving a plul rality of discrete uni-directional potentialsfrom a radio signal, means for algebraically combining the effects of said potentials for control'purposes,

and regenerative means for reinforcing the Yeffect of one of said potentials and opposing theeifect of another.

7. In combination, means for deriving a plu- ',rality of discrete uni-directional potentials from al radio signal, means for algebraically combining the effects of said potentials for control purposes,

rectional potential providing means fresponsive 'during the tuning of the receiver toward resonance with the carrier wave vof a desired signal and relatively less responsive at the resonance point, uni-directional potential supplying means having maximum response at the resonance point, means for utilizing the potential from the first-mentioned potential supplying means to re- 'Vtransmission eiciency adjustment.

in fo'rce the Yaction "of the biasing means, and

means for utilizing the'potential Yfrom the sec- 'ond-mentioned potential 1 supplying means forV opposing the eie'ct of the potential from the iirst mentioned' rpotential supplying means.

9. The inventionrset forth in claim 8, further characterized in that regenerative means are provided for regeneratively reinforcing'the action of the potential supplying meansfhaving maximum response at resonance, whereby Athe biasing potential is abruptly removed from vthe biased stages as the receiver is tuned to resonance with a desired signal.

10. In a radio'receiving system, means for suppressing inter-station noises, a signal responsive device for controlling the. action of said noise Y suppressing means, a second signal responsive device for controlling saidl noise suppressing means, and connections whereby the action of the y two signal responsive devicesis in the opposite sense. 11. In combination, means for deriving ajplurality of discrete uni-directional potentials froml a Vradio signal, means-for utilizing .the algebraic y sum of said potentials for controlling Ythe con-V ductivity of an impedance device, and additional lmeans for regenerativelyV utilizing the signal to reinforce the effect of Yone of said potentials and `Vopposethe effect of another.

' the action of the rst named means and means' "including a circuit having relatively less damp- 'ing for nullifying the action of thesecond named 13. In a tunable radio receiver, an amplifying stage, means forbiasing said stage to substantially the cut-off potential during' the absence of meansv when signals from adesired station are a signal, meansA functioning at maximum eiiciency at reach side of resonance with an incorn-- ingrsignal to provide a uni-directional potential,"A means functioning at maximum efficiency at resonance with an incoming signal to providea second uni-directional potential and means for utilizing said potentials, respectively, for reinforcing the action-of the biasing means and for nullifying the action-of the biasing means. Y n

14. In a radio receiver, at least twocascaded signal transmission tubes,an electron discharge tube of the' type comprisingra diode section and a anode,a signal path connecting one'of the transmission tubes to said diode.sectiona circuit op# eratively associated withthe diode section for Vdeveloping a gain control voltage from received@ signalsgmeans for applying the control voltages' a section including at leastafcathode, gridv and-"55 to a gain control point in the receiver, means`,'re

sponsive to; variations in the spacecurrent'ow of said discharge tube cathodecircuit, for adjustcascade'd tube, at :least two rectifier networks,f means for impressing signals on the networks -to derive uni-directional potentials therefrom, and means `for differentially applying the potentials to said grid thereby toV controlysaidn 15. In a' multi-stage radio receiver, Vaninter` tube transformer having a tuned primary wind-f ing and a tuned secondaryv winding, means lfor ing the' signal transmission efficiency of the otherM Ving potentials developed across the secondary of the transformer for rendering inactive theblocking means and means energizedfrom the primary of the transformer for counteracting the action of the means for utilizing the potentials developed across the secondary thereof, whereby the apparent selectivity of the receiver with respect to the blocking function is increased over the selectivity of the transformer-per se.

16. In a radio receiver, a main signal amplifying channel including a plurality of tuned stages and having a predetermined degree of selectivity, means for blocking at least one of. said stages during absence of a signal in said channel, means whereby another of said stages provides a uni-directional potential in response to an incoming signal, means extraneous to said channel for providing al second uni-directional Vpotential in response Vto an incoming signal in said channel and means for utilizing the algebraic sum of said potentials for rendering ineffective the action of the stage blocking means, the extraneous potential providingmeans having a response characteristic exhibiting a pronounced dip at a frequency whereat the response of the potential-supplying stage is'maximum, whereby the apparent selectivity of the receiver with reispect to the function of stage-blocking is greatly increased over the selectivity of the signal amplifying channelV per se.

V17 In combination with a high frequency amplier whose gain is to be controlled, a gain control tube comprising a diode section and a triode section having a common cathode, an impedance connected in circuit with said diode section, a

direct current connection jbetween -apoint on -said impedance and the grid' circuit of said amplier, a gecond impedance in the cathode circuit of said gain control tube, a second amplifier following the first amplifier, a direct current connection between the cathode side of said second impedance and the cathode circuit of said second amplifier, a second control tube including a cathode and at least vtwo anodes, a pair of tuned circuits resonant to the operating signal frequency connected between each of said last named anodes and the cathode of said second control tube, one of said tuned circuits being coupled between the output circuit of vsaid amplifier and the other tuned circuit, and a direct current connection between the grid of said triode section and a Y point in they diode circuit including said other tuned circuit.

18. In combination with a high frequency amplifier whose gain is to be controlled, a gain control tube comprising a diode section and a triode section having a common cathode, an impedance connected in circuit with said diode section, a direct current connection between a point on said impedance and the grid circuit of said amplifieiya-second impedance in the cathode circuit ofY said gain control tube, a second amplifier following the first amplifier, a direct current connection between the cathode side of Ysaid second impedance and the cathode circuit of said second amplifier, a second-control tube including a cathode and at least two anodes, a pair of tuned circuits resonant to the operating signal vfrequency connected between each of said last namedV anodesand the cathode of said second ycontrol tube, one of saidrtuned circuits being coupled between the output circuit of saidramplifier and the other tuned circuit, and a direct current connection between the grid of said triode section and a point in the diode circuit including vsaid other tuned circuit, a demodulator,'means lator, and a direct current connection between 4 the output electrode of saidlast named rectifier rand the grid of said triode section.

19. In combination with a high frequency amplifier whose gain is to be controlled, a gain control tube comprising a diode section and a triode f section having a common cathode, an impedance connected in circuit with said diode sectionVa direct current connection between a point on said impedance and the grid circuit of said amplifier, a second impedancerin the cathode circuit of said gain control tube, a second amplifier following the first amplifier, a direct currentconnection between the cathode side of said second impedance and the cathode circuit of said second amplifier, a second control tube including a cathode and at least two anodes, a pairof tuned circuits resonant to the operating signal frequency connected between each of said last named anodes and the cathode of said second control tube, Yone of said tuned circuits being coupled between the output circuit of said amplifier and the other tuned circuit, and a direct current connection between the grid of said triode section and a point in the diode circuit including said other4 tuned circuit, and a diode having a tuned circuit resonant to said signal frequency connected between the anode and cathode thereof, said last tuned circuit being coupled-to the output of said amplifier, a third' impedance inthe circuit of said last named diode, means for ,con-

necting a point of negative direct current potential on said third impedance to the grid of said triode section. Y

20. In combination with a high frequency amplifier whose gain is to be controlled, a gain con-,-

trol tube comprising a diode section and a triode section having a common cathode, an impedance connected in circuit with said `diode section, a direct current connection between a point on said impedance and the grid circuit of said amplifler, a second impedance linthe cathode circuit of said gain control tube, a second amplifier following the first amplifier, a ydirect current connection'between the cathode side of saidlsecond impedance and the cathode circuit of said second amplifier, a second control tube including a cathode and at least two anodes, a pair of tuned circuits resonant to the operating signal frequency connected between each of Said last named anodes and the cathode of said second control tube, one of said tuned circuits being coupled between the output circuit of said amplifier and the other tuned circuit, and a direct current connection between the grid of said triode section and a point in the diode circuit including said other tuned,-

circuit, and -a diode having a tuned circuit resonant to said signal frequency connected between the anode and cathodethereof, said last tuned circuit being coupled to the output of said ampli'- fier, a third impedance in the circuit of said last named diode, means for connecting a point .of negativeV direct current potential on said third impedance to the grid of said triode section, and

additional means for impressing the audio component of a rectified signal voltage developed,U

across said third impedance upon an audio frequency amplifier.

2l. In combination with a high frequency amplifier whose gain is to be controlled, a gain con- Y trol tube comprising a diode section and a triode;

section having a common cathode, an impedance connected in circuit with said diode section, a direct current connection between a point on said impedance and the grid circuit of said amplier, a second impedance in the cathode circuit of said gain control tube, a second amplier following the rst amplier, a direct current connection between the cathode side of said second impedance and the cathode circuit of said second amplier, a second control tube including a cathode and at least two anodes, a pair of tuned circuits resonant to the operating signal frequency connected between each of said last named anodes and the cathode of said second control tube, one of said tuned circuits being coupled between the output circuit of said amplier and the other tuned circuit, and a direct current connection between the grid of said triode section and a point in the diode circuit including said other tuned circuit, a resistor in circuit with said diode section including said other tuned circuit, a second tion, a rectier having its input electrode connected through a signal path to the Voutput of said amplifier, and a direct current connection including said two resistors to the grid of said triode section. Y

22. In a high frequency signallingfsystem, means for adjusting the signal transmission eiciency of said system, means for controllingY the signal transmission through the system in an opposite sense to said adjustment, means including circuits of different selectivity for providing at least two `uni-directional potentials whose magnitude and polarity are dependent upon a desired signal, means for utilizing said potentials to control the eiectiveness of operation of said controlV means, and additional means, responsive to said desired signal, for increasing the rate of effectiveness of said control means.

Y GEORGE L.,BEERS.

resistor in circuit with the remaining diode sec-V zov 

